In our urban society a majority of utilities are routed via underground conduits. Access to the conduits is provided at key locations by way of manholes whereby workmen may descent into the conduits and repair or add utility facilities. This work is normally time consuming and the workmen must remain in the underground conduits for extended periods of time. The conduits are not ventilated and therefore noxious and poisonous gases may accumulate therein and create an atmosphere which is hazardous to their health. Therefore, it has been a practice to provide ventilating air to a manhole by way of a small, portable, engine driven squirrel cage type fan. These fans are usually carried by service trucks when not in use and deployed by placing them on the ground adjacent to the manhole being serviced. This results in the fan scavenging noxious gases from the surface, such as exhaust fumes from the engine driving the fan, and forcing them into the area being serviced where they contribute to the unhealthy atmosphere rather than improve it. Furthermore, this air can be extremely cold in the winter and hamper the servicemen. This is overcome by attaching a propane heater to the blower fan housing. This provides heated air but it creates logistics problems in setting up the bulky heating equipment and fuel source near the manhole in an area which may be a crowded city street.
Servicemen working in underground conduits require compressed air and electricity to drive their tools and provide a means to illuminate the work area. This is usually supplied by an air compressor and electrical generator, both of which are positioned on the surface near the manhole. This results in a large amount of equipment deployed about a manhole and creates significant traffic disruptions. Furthermore, the time required to deploy the various components required to service workmen in a manhole greatly increases the cost for accomplishing a predetermined job in a conduit.
These drawbacks have been partially overcome by systems such as that disclosed in U.S. Pat. No. 3,672,445 issued to T. Carson on July 27, 1972. This patent discloses a truck mounted system which utilizes the prime mover driven generator to provide electric power for an electrically driven air compressor and high-volume low-pressure air ventilation system. The Carson system also includes a heat exchanger wherein hot water from the truck engine heats the ventilating air supplied via the low-pressure, high-volume portion of the system.
Truck mounted systems such as Carson U.S. Pat. No. 3,672,445 must be permanently installed in the vehicle due to the water and electrical interconnections between the system and the vehicle engine. Therefore a truck must be designated as a manhole service truck and this results in a significant capital expenditure for each manhole support service system. Operation of the system is also uneconomical because it requires that the engine of the truck be run constantly while the service module is in operation.
This is costly not only in fuel consumed but also in the useful life of the truck engine since it is being operated in an environment for which it was not originally designed.
An underground service module presented in copending U.S. patent application Ser. No. 968,790, filed Dec. 12, 1978, in the name of Miles T. Carson has solved many of the problems existing in prior art service systems. However, the underground service module disclosed in the copending patent application while providing many needed features, fails to permit adjustment of air volume and mean air temperature in the conditioned air circuit. That system also fails to include safety features which will permit remote control of the conditioned air and the heat exchange systems are not optimized for maximum efficiency.